You need to talk to people who are having problems viewing the site(s). Are they unable to view certain things, or everything? If you can talk to them, have them ping the domain name in question from a dos prompt:

Code:

ping domain.com

If they can see ANYTHING from the domain on the new server, their DNS is working fine and the problem is elsewhere.

I can't imagine an ISP waiting months to update their DNS but it might happen, and it's completely out of your control. Contact their ISP and slap them on the back of their head